1. Field of the Invention
This invention relates to methods of manufacturing a semiconductor device. More specifically, the invention relates to a method of manufacturing a semiconductor device having a trench.
2. Related Art
A semiconductor acceleration or angular velocity sensor typically includes a small comb-shaped beam structure formed on a substrate for detecting a physical quantity. The comb-shaped structure is formed so that the physical quantity is detected by way of electrostatic force. The distance between each comb-like element must be narrow, and the thickness of the structure must be set to a particular value in order to downsize the sensor and achieve a high detection performance.
The typical comb-shaped structure is created by forming a mask on the substrate and by dry-etching the substrate through the mask. The aspect ratio of the trench, which is the depth of a trench with respect to an opening width of the trench, is typically set to a large number.
A large trench aspect ratio can be a limitation from a process technology viewpoint. In the case of normal dry etching, if high anisotropy anisotropic etching (in which an etching rate for a vertical direction of the substrate is higher than that for the other directions) is used, the etching is typically gradually advanced in a width direction of the trench from a surface portion of the substrate in the trench. Therefore, when the etching is performed for a long time, a sectional shape of the trench may be a V-shape because the trench width is gradually enlarged, so that the aspect ratio is saturated at a certain degree.
A method of solving the above problem is disclosed in U.S. Pat. No. 5,501,893 to Laermer. Laermer discloses a dry-etching method including two steps including (1) plasma etching having a high anisotropy; and (2) deposition of a polymer-based thin film.
During thin film deposition, the polymer-based thin film is deposited on each sidewall of the etched trenches in order to serve as a protection film and to prevent the sidewall of the trench from being etched during plasma etching. In this manner, it can prevent the opening of the trench from being etched toward the opening width direction and improve an upper limitation of the aspect ratio of the trench shape compared to normal dry etching.
However, this method fails to completely protect the sidewalls from being etched because the trench width is slightly enlarged. The limitation of the aspect ratio is also present. FIG. 9 shows the relationship between process time and aspect ratio when the trench is experimentally processed in a reactive dry etching (RIE) apparatus based on the above-described method. As shown from this figure, the aspect ratio never exceeds 25 even when the etching is performed for a long time.
Another method to solve the above-described inconvenience is disclosed in U.S. Pat. No. 5,658,472 to Bartha. Bartha also divides an etching process into an etching step and a step for forming a protection film on a sidewall of a trench, and alternately performs these two steps. In this technology, the step for forming the protection film is performed by depositing a thermal oxide film (SiO.sub.2) in a chamber different from that in which the etching is performed, or by depositing a thin ice film in the chamber in which the etching is performed. In comparison to the polymer-based film, the aspect ratio is improved because the films disclosed in Bartha have durability for sidewall etching.
However, it would take a long time to unload the substrate and to form the thermal oxide film every time the protection film is deposited. Furthermore, the temperature of the substrate needs to rise and fall after every unloading. Therefore, it is not preferable from a viewpoint of process throughput. Also, the substrate needs to be below the freezing point during the etching in order to use the ice film as the protection film. These limitations cause the apparatus to be complicated.